Here is the full list of all 21 Iridescence achievements.
You defeated the 1st Boss.
You defeated the 2nd Boss.
You defeated the 3rd Boss.
You defeated the 4th Boss.
You defeated the 5th Boss.
Defeat any boss with at least 50% of time left on the clock.
Get A Rank or better on any level.
Get S Rank or better on any level.
Get S++ Rank on any level.
Beat game on medium or hard.
Beat game on hard.
Beat the game scoring 500,000 or more points on any difficulty.
Beat all challenge levels
Beat all survival levels.
Have 1,000 credits in the bank.
Last 60 seconds or more on any survival level.
Beat any level without shooting at all.
Buy a challenge level.
Gain an extra life.
Complete Tutorial.
Get a x8 multiplier.